Temperature-induced decoupling of phycobilisomes from reaction centers

How electron tunneling and uphill excitation energy transfer support photochemistry in Halomicronema hongdechloris

Halomicronema hongdechloris, the first cyanobacterium reported to produce the red-shifted chlorophyll f (Chl f) upon acclimation to far-red light, demonstrates remarkable adaptability to diverse light conditions. The photosystem II (PS II) of this organism undergoes reversible changes in its Chl f content, ranging from practically zero under white-light culture conditions to a Chl f: Chl a ratio of up to 1:8 when exposed to far-red light (FRL) of 720-730 nm for several days. Our ps time- and wavelength-resolved fluorescence data obtained after excitation of living H. hongdechloris cells indicate that the Soret band of a far-red (FR) chlorophyll involved in charge separation absorbs around 470 nm. At 10 K, the fluorescence decay at 715-720 nm is still fast with a time constant of 165 ps indicating an efficient electron tunneling process. There is efficient excitation energy transfer (EET) from 715-720 nm to 745 nm with the latter resulting from FR Chl f, which mainly functions as light-harvesting pigment upon adaptation to FRL. From there, excitation energy reaches the primary donor in the reaction center of PS II with an energetic uphill EET mechanism inducing charge transfer. The fluorescence data are well explained with a secondary donor PD1 represented by a red-shifted Chl a molecule with characteristic fluorescence around 715 nm and a more red-shifted FR Chl f with fluorescence around 725 nm as primary donor at the ChlD1 or PD2 position.

Sponge-derived Ageladine A affects the in vivo fluorescence emission spectra of microalgae

In several marine hosts of microalgae, fluorescent natural products may play an important role. While the ecological function of these compounds is not well understood, an interaction of these molecules with the photosynthesis of the symbionts has been suggested. In this study, the effect of Ageladine A (Ag A), a pH-dependent fluorophore found in sponges of the genus Agelas, on microalgal fluorescence was examined. The spectra showed an accumulation of Ag A within the cells, but with variable impacts on fluorescence. While in two Synechococcus strains, fluorescence of phycoerythrin increased significantly, the fluorescence of other Synechococcus strains was not affected. In four out of the five eukaryote species examined, chlorophyll a (Chl a) fluorescence intensity was modulated. In Tisochrysis lutea, for example, the position of the fluorescence emission maximum of Chl a was shifted. The variety of these effects of Ag A on microalgal fluorescence suggests that fluorophores derived from animals could play a crucial role in shaping the composition of marine host/symbiont systems.

A new bioenergetic and thermodynamic approach to batch photoautotrophic growth of Arthrospira (Spirulina) platensis in different photobioreactors and under different light conditions

Photobioreactor configuration, mode of operation and light intensity are known to strongly impact on cyanobacteria growth. To shed light on these issues, kinetic, bioenergetic and thermodynamic parameters of batch Arthrospira platensis cultures were estimated along the time at photosynthetic photon flux density (PPFD) of 70μmolm(-2)s(-1) in different photobioreactors with different surface/volume ratio (S/V), namely open pond (0.25cm(-1)), shaken flask (0.48cm(-1)), horizontal photobioreactor (HoP) (1.94cm(-1)) and helicoidal photobioreactor (HeP) (3.88cm(-1)). Maximum biomass concentration and productivity remarkably increased with S/V up to 1.94cm(-1). HoP was shown to be the best-performing system throughout the whole runs, while HeP behaved better only at the start. Runs carried out in HoP increasing PPFD from 40 to 100μmolm(-2)s(-1) revealed a progressive enhancement of bioenergetics and thermodynamics likely because of favorable light distribution. HoP appeared to be a promising configuration to perform high-yield indoor cyanobacterial cultures.

Identification of biochemical association of phycobilisome with photosystems in cyanobacterial state transition

State transition is a short-term balance mechanism of energy distribution between photosystem II (PSII) and PSI. Although light-induced state transition in cyanobacteria has been suggested to depend completely on the phycobilisome (PBS) movement between PSII and PSI, the biochemical evidence has not been clearly shown. In this study, we locked the association of PBS with PSII or PSI using glycinebetaine when cells attain State 1 or 2 by exposure to light of blue or green, respectively. Subsequently, the PBS-reaction centers were resolved by blue native polyacrylamide gel electrophoresis and two-dimensional electrophoresis, and then identified by western blot analysis. The results showed that in wild-type (WT) Synechocystis sp. strain PCC 6803, the PBS core always co-migrates with the PSII dimer during light-induced State 1-State 2 transition, but its rod leaves the PSII dimer in State 2 regardless of its co-migration in State 1. In the light-induced State 2, the co-migration of PBS rod with PSI trimer was observed in WT, but not in ΔndhB (M55), a State-2-transition-deficient mutant. This study first provided the biochemical evidence for the association of PBS with photosystems during cyanobacterial state transition.

Connecting thermal physiology and latitudinal niche partitioning in marine Synechococcus

Marine Synechococcus cyanobacteria constitute a monophyletic group that displays a wide latitudinal distribution, ranging from the equator to the polar fronts. Whether these organisms are all physiologically adapted to stand a large temperature gradient or stenotherms with narrow growth temperature ranges has so far remained unexplored. We submitted a panel of six strains, isolated along a gradient of latitude in the North Atlantic Ocean, to long- and short-term variations of temperature. Upon a downward shift of temperature, the strains showed strikingly distinct resistance, seemingly related to their latitude of isolation, with tropical strains collapsing while northern strains were capable of growing. This behaviour was associated to differential photosynthetic performances. In the tropical strains, the rapid photosystem II inactivation and the decrease of the antioxydant β-carotene relative to chl a suggested a strong induction of oxidative stress. These different responses were related to the thermal preferenda of the strains. The northern strains could grow at 10 °C while the other strains preferred higher temperatures. In addition, we pointed out a correspondence between strain isolation temperature and phylogeny. In particular, clades I and IV laboratory strains were all collected in the coldest waters of the distribution area of marine Synechococus. We, however, show that clade I Synechococcus exhibit different levels of adaptation, which apparently reflect their location on the latitudinal temperature gradient. This study reveals the existence of lineages of marine Synechococcus physiologically specialised in different thermal niches, therefore suggesting the existence of temperature ecotypes within the marine Synechococcus radiation.

Mobility of photosynthetic proteins

The mobility of photosynthetic proteins represents an important factor that affects light-energy conversion in photosynthesis. The specific feature of photosynthetic proteins mobility can be currently measured in vivo using advanced microscopic methods, such as fluorescence recovery after photobleaching which allows the direct observation of photosynthetic proteins mobility on a single cell level. The heterogeneous organization of thylakoid membrane proteins results in heterogeneity in protein mobility. The thylakoid membrane contains both, protein-crowded compartments with immobile proteins and fluid areas (less crowded by proteins), allowing restricted diffusion of proteins. This heterogeneity represents an optimal balance as protein crowding is necessary for efficient light-energy conversion, and protein mobility plays an important role in the regulation of photosynthesis. The mobility is required for an optimal light-harvesting process (e.g., during state transitions), and also for transport of proteins during their synthesis or repair. Protein crowding is then a key limiting factor of thylakoid membrane protein mobility; the less thylakoid membranes are crowded by proteins, the higher protein mobility is observed. Mobility of photosynthetic proteins outside the thylakoid membrane (lumen and stroma/cytosol) is less understood. Cyanobacterial phycobilisomes attached to the stromal side of the thylakoid can move relatively fast. Therefore, it seems that stroma with their active enzymes of the Calvin-Benson cycle, are a more fluid compartment in comparison to the rather rigid thylakoid lumen. In conclusion, photosynthetic protein diffusion is generally slower in comparison to similarly sized proteins from other eukaryotic membranes or organelles. Mobility of photosynthetic proteins resembles restricted protein diffusion in bacteria, and has been rationalized by high protein crowding similar to that of thylakoids.

The dynamic behavior of phycobilisome movement during light state transitions in cyanobacterium Synechocystis PCC6803

Light state transition is a physiological function of oxygenic organisms to balance the excitation of photosystem II (PSII) and photosystem I (PSI), hence a prerequisite of oxygen-evolving photosynthesis. For cyanobacteria, phycobilisome (PBS) movement during light state transition has long been expected, but never observed. Here the dynamic behavior of PBS movement during state transition in cyanobacterium Synechocystis PCC6803 is experimentally detected via time-dependent fluorescence fluctuation. Under continuous excitation of PBSs in the intact cells, time-dependent fluorescence fluctuations resemble "damped oscillation" mode, which indicates dynamic searching of a PBS in an "overcorrection" manner for the "balance" position where PSII and PSI are excited equally. Based on the parallel model, it is suggested that the "damped oscillation" fluorescence fluctuation is originated from a collective movement of all the PBSs to find the "balance" position. Based on the continuous fluorescence fluctuation during light state transition and also variety of solar spectra, it may be deduced that light state transition of oxygen-evolution organisms is a natural behavior that occurs daily rather than an artificial phenomenon at extreme light conditions in laboratory.

Estimation of relative contribution of "mobile phycobilisome" and "energy spillover" in the light-dark induced state transition in Spirulina platensis

Previously, it was clarified that phycobilisome (PBS) mobility and energy spillover were both involved in light-to-dark induced state transitions of intact Spirulina platensis cells. In this work, by taking advantage of the characteristic fluorescence spectra of photosystem I (PSI) trimers and monomers as indicators, the relative contributions for the "mobile PBS" and "energy spillover" are quantitatively estimated by separating the fluorescence contribution of PBS mobility from that of PSI oligomeric change. Above the phase transition temperature (TPT) of the membrane lipids, the relative proportion of the contributions is invariable with 65% of "mobile PBS" and 35% of "energy spillover". Below TPT, the proportion for the "mobile PBS" becomes larger under lowering temperature even reaching 95% with 5% "energy spillover" at 0 degrees C. It is known that lower temperature leads to a further light state due to a more reduced or oxidized PQ pool. Based on the current result, it can be deduced that disequilibrium of the redox state of the PQ pool will trigger PBS movement instead of change in the PSI oligomeric state.

The origin of the temperature-induced fluorescence fluctuation in Spirulina platensis: temperature-sensitive mobility of PQ molecules

Temperature effects on state transitions have been studied in the cyanobacterium Spirulina platensis. At lower temperatures the time to reach completion took longer and the extent of the state transitions was larger. Effects were limited to the temperature range below the phase transition temperature of the membrane lipids. In the presence of the artificial electron acceptor phenyl-1,4-benzoquinone (PBQ) state transitions became completely temperature-independent. State transitions induced by a change in the light climate or in darkness by a switch from aerobic to anaerobic conditions responded similar to temperature; the occurrence of state transitions solely by a change of the temperature has been excluded. Our conclusion is that the temperature-dependent mobility of plastoquinone molecules in the thylakoid membranes is the intrinsic cause of temperature effects on state transitions.

Fluorescence induction in the phycobilisome-containing cyanobacterium Synechococcus sp PCC 7942: Analysis of the slow fluorescence transient

At room temperature, the chlorophyll (Chl) a fluorescence induction (FI) kinetics of plants, algae and cyanobacteria go through two maxima, P at approximately 0.2-1 and M at approximately 100-500 s, with a minimum S at approximately 2-10 s in between. Thus, the whole FI kinetic pattern comprises a fast OPS transient (with O denoting origin) and a slower SMT transient (with T denoting terminal state). Here, we examined the phenomenology and the etiology of the SMT transient of the phycobilisome (PBS)-containing cyanobacterium Synechococcus sp PCC 7942 by modifying PBS-->Photosystem (PS) II excitation transfer indirectly, either by blocking or by maximizing the PBS-->PS I excitation transfer. Blocking the PBS-->PS I excitation transfer route with N-ethyl-maleimide [NEM; A. N. Glazer, Y. Gindt, C. F. Chan, and K.Sauer, Photosynth. Research 40 (1994) 167-173] increases both the PBS excitation share of PS II and Chl a fluorescence. Maximizing it, on the other hand, by suspending cyanobacterial cells in hyper-osmotic media [G. C. Papageorgiou, A. Alygizaki-Zorba, Biochim. Biophys. Acta 1335 (1997) 1-4] diminishes both the PBS excitation share of PS II and Chl a fluorescence. Here, we show for the first time that, in either case, the slow SMT transient of FI disappears and is replaced by continuous P-->T fluorescence decay, reminiscent of the typical P-->T fluorescence decay of higher plants and algae. A similar P-->T decay was also displayed by DCMU-treated Synechococcus cells at 2 degrees C. To interpret this phenomenology, we assume that after dark adaptation cyanobacteria exist in a low fluorescence state (state 2) and transit to a high fluorescence state (state 1) when, upon light acclimation, PS I is forced to run faster than PS II. In these organisms, a state 2-->1 fluorescence increase plus electron transport-dependent dequenching processes dominate the SM rise and maximal fluorescence output is at M which lies above the P maximum of the fast FI transient. In contrast, dark-adapted plants and algae exist in state 1 and upon illumination they display an extended P-->T decay that sometimes is interrupted by a shallow SMT transient, with M below P. This decay is dominated by a state 1-->2 fluorescence lowering, as well as by electron transport-dependent quenching processes. When the regulation of the PBS-->PS I electronic excitation transfer is eliminated (as for example in hyper-osmotic suspensions, after NEM treatment and at low temperature), the FI pattern of Synechococcus becomes plant-like.

Demonstration of phycobilisome mobility by the time- and space-correlated fluorescence imaging of a cyanobacterial cell

The cell-wide mobility of PBSs was confirmed by synchronously monitoring the fluorescence recovery after photobleaching (FRAP) and the fluorescence loss in photobleaching (FLIP). On the other hand, a fluorescence recovery was still observed even if PBSs were immobile (PBSs fixed on the membranes by betaine and isolated PBSs fixed on the agar plate) or PBS mobility was unobservable (cell wholly bleached). Furthermore, it was proved that some artificial factors were involved not only in FRAP but also in FLIP, including renaturation of the reversibly denatured proteins, laser scanning-induced fluorescence loss and photo-damage to the cell. With consideration of the fast renaturation component in fluorescence recovery, the diffusion coefficient was estimated to be tenfold smaller than that without the component. Moreover, it was observed that the fluorescence intensity on the bleached area was always lower than that on the non-bleached area, even after 20 min, while it should be equal if PBSs were mobile freely. Based on the increasing proportion of the PBSs anti-washed to Triton X-100 (1%) with prolonged laser irradiation to the cells locked in light state 1 by PBQ, it was concluded that some PBSs became immobile due to photo-linking to PSII.

The influence of acetyl phosphate on DspA signalling in the Cyanobacterium Synechocystis sp. PCC6803

BACKGROUND

The dspA (hik33) gene, coding for a putative sensory histidine kinase, is conserved in plastids (ycf26) and cyanobacteria. It has been linked with a number of different stress responses in cyanobacteria.

RESULTS

We constructed an insertional mutant of dspA (ycf26) in Synechocystis 6803. We found little phenotypic effect during nitrogen starvation. However, when the mutation was combined with deletion of the pta gene coding for phosphotransacetylase, a more significant phenotype was observed. Under nitrogen starvation, the pta/dspA double mutant degrades its phycobilisomes less than the wild type and still has about half of its chlorophyll-protein complexes.

CONCLUSION

Our data indicates that acetyl-phosphate-dependent phosphorylation of response regulator(s) overlaps with DspA-dependent signalling of the degradation of chlorophyll-protein complexes (and to a lesser extent phycobilisomes) in Synechocystis 6803.

Spectroscopic investigation on the energy transfer process in photosynthetic apparatus of cyanobacteria

In this work, we employ cyanobacteria, Spirulina platensis, and separate their photosynthetic apparatus, phycobilisome (PBS), thylakoid membrane and phycobilisome-thylakoid membrane complex. The steady state absorption spectra, fluorescence spectra and corresponding deconvoluted spectra and picosecond time-resolved spectra are used to investigate the energy transfer process in phycobilisome-thylakoid membrane complex. The results on steady state spectra show chlorophylls of the photosystem II are able to transfer excitation energy to phycobilisome with Chla molecules selectively excited. The decomposition of the steady state spectra further suggest the uphill energy transfer originate from chlorophylls of photosystem II to cores of phycobilisome, while rods and cores of phycobilisome cannot receive energy from the chlorophylls of photosystem I. The time constant for the back energy transfer process is 18 ps.

Light-induced excitation energy redistribution in Spirulina platensis cells: “spillover” or “mobile PBSs”?

State transitions induced by light and redox were investigated by observing the 77 K fluorescence spectra for the intact cells of Spirulina platensis. To clarify if phycobilisomes (PBSs) take part in the state transition, the contributions of PBSs to light-induced state transition were studied in untreated cells and the cells treated by betaine which fixed PBSs firmly on the thylakoid membranes. It was observed that the betaine-treated cells did not show any light-induced state transition. This result definitely confirmed that the light-induced excitation energy regulation between the two photosystems is mainly dependent on a spatial movement of PBSs on the thylakoid membranes, which makes PBS cores partially decoupled from photosystem II (PSII) while PBS rods more strongly coupled with photosystem I (PSI) during the transition from state 1 to state 2. On the other hand, an energy exchange between the two photosystems was observed in both untreated and betaine-treated cells during redox-induced state transition. These observations suggested that two different mechanisms were involved in the light-induced state transition and the redox-induced one. The former involves only a physical movement of PBSs, while the latter involves not only the movement of PBS but also energy spillover from PSII to PSI. A model for light-induced state transition was proposed based on the current results as well as well known knowledge.

Probing connection of PBS with the photosystems in intact cells of Spirulina platensis by temperature-induced fluorescence fluctuation

Temperature-dependent fluorescence for intact cells of cyanobacterium Spirulina platensis was detected to search for the connection of the phycobilisome (PBS) with Photosystem I (PSI) and Photosystem II (PSII). Some interesting results were obtained from the deconvoluted fluorescence components of C-phycocyanin (C-PC), allophycocyanin (APC), PSI and PSII as well as the fluorescence spectra of the intact cells at room temperature (RT=25 degrees C) and 0 degrees C. It was observed that, compared to those at RT, both of the fluorescence components for PSI and APC increased, whereas those for PSII and C-PC decreased at 0 degrees C with excitation at 580 nm, that is, the fluorescence for C-PC is not synchronous with that for APC, and the fluorescence fluctuation for PSI is not synchronous with that for PSII. On the other hand, the decrease in C-PC fluorescence is synchronous with the increase in PSI fluorescence, and the increase in APC fluorescence is synchronous with the decrease in PSII fluorescence. Therefore, it can be readily deduced that PBS should be coupled not only with PSII through the terminal acceptors in the APC core but also with PSI through C-PC in PBS rods at physiological condition, while at 0 degrees C, a migration of a PBS makes the APC partially detached from PSII but the C-PC more efficiently coupled with PSI. The results provide good evidences for "mobile PBS" model and "parallel connection" model but not for the "spillover" model.